The present invention relates to an aircraft nacelle comprising an improved air intake.
As shown in FIG. 1, an aircraft nacelle 10 comprises an air intake 12 at the front allowing an air flow to be channeled in the direction of an engine assembly, a first portion of the incoming air flow, referred to as the primary flow, passing through the turbojet to take part in the combustion process, the second portion of the air flow, referred to as the secondary flow, being entrained by a fan 14 and flowing in an annular conduit delimited by the internal wall of the nacelle and the external wall of the turbojet.
In the description below, the longitudinal direction corresponds to the direction of the axis of rotation 16 of the fan 14. A longitudinal plane is a plane that passes through the axis of rotation 16. A transverse plane is a plane perpendicular to the axis of rotation 16.
As illustrated in FIG. 2, the air intake 10 comprises a lip 18, the surface whereof in contact with the aerodynamic flows is extended on the inside of the nacelle by an internal conduit 20 that is substantially circular and on the outside of the nacelle by an external wall 22 that is substantially circular. The lip 18 allows an air flow 24 to be channeled to the inside of the internal conduit 20.
In a longitudinal plane, the air intake 12 has a section with a portion that is substantially straight at the level of the internal conduit 20, a portion that is substantially straight at the level of the external wall 22 and a curved intermediate portion with a small bending radius at the level of the lip 18.
This curved intermediate portion has a vertex A which corresponds to the foremost point of the lip 18. This vertex A describes an approximately circular profile 26 along the circumference of the nacelle 10, as illustrated in FIG. 1. This profile 26 corresponds to the leading edge of the air intake.
According to the prior art, this profile 26 is inscribed in a plane 28. This plane 28 is preferably not perpendicular to the axis of rotation 16 of the fan 14 and forms an angle in the order of 75 to 105° with this axis of rotation 16, such that the vertex of the air intake situated at the top of the nacelle, which corresponds to an angular position of 0°, is further forward than the vertex of the air intake situated at the bottom of the nacelle, which corresponds to an angular position of 180°.
FIG. 3A shows the opened-out profile 26 described by the vertex A in relation to a reference plane perpendicular to the axis of rotation 16 of the fan 14.
Because the plane 28 is inclined relative to the axis of rotation 16, the profile 26 approximately describes a single sine curve with an amplitude E which corresponds to the gap in the longitudinal direction between the vertex of the air intake situated at the top of the nacelle and that situated at the bottom.
FIGS. 3B and 3C show a transverse section through the internal conduit 20 at a point B of the air intake and at a point C of the air intake.
Points B and C each approximately describe a circle of radius R and radius R′, respectively, as illustrated in FIGS. 3B and 3C.
When the air flow penetrates the air intake 12, depending on the flight phase of the aircraft (climbing, cruising, descent), the air flow may break away at the junction zone between the lip 18 and the internal conduit 20, causing an increase in thickness of the boundary layer 32 on the surface of the internal conduit 20, as illustrated in FIG. 2. Since the thickness of the boundary layer 32 is greater than the clearance allowed between the internal conduit 20 and the ends of the blades 34, said blades are not in a longitudinal laminar flow, but interfere with the boundary layer 32. The interaction between the ends of the blades 34 and the turbulent flows of the boundary layer 32 generates a noise source.
In order to limit the impact of unwanted noise, techniques have been developed to reduce noise, notably by providing panels or casings at the internal conduit walls 20, said panels or casings being intended to absorb part of the noise energy, particularly by applying the principle of Helmholtz resonators.
Even if these panels or casings are efficient, they simply attenuate the noises emitted by certain engine sources and are not intended to limit their emergence.